The present invention relates to well logging, and in particular to improvements in a borehole logging tool referred to as a borehole televiewer, or BHTV. Tools of this type are described, for example, in U.S. Pat. No. 3,369,626 (Zemanek, Jr., issued Feb. 20, 1968), U.S. Pat. No. 3,478,839 (Zemanek, Jr., issued Nov. 18, 1969), U.S. Pat. No. 4,463,378 (Rambow, issued July 31, 1984), and U.S. Pat. No. 4,601,024 (Broding, issued July 15, 1986).
In general, borehole televiewer logging tools operate acoustically by periodically pulsing a rotating acoustic transducer to emit a sequence of acoustical pulses directionally into the borehole toward the borehole wall, and analyzing the echos which are reflected back to the tool. The amplitude of the reflected signal may then be displayed on a cathode ray tube, the display sometimes being photographed for future reference. Typically, the display represents a map of the borehole wall split along the north direction and laid out flat. Alternatively, a polar display may be produced, in which case the radius of the circular trace is determined by the time-of-flight of the acoustic pulse, thus presenting a cross-sectional profile of the borehole. Another display, similar to the amplitude display, is modulated by the time-of-flight signal rather than the amplitude signal. The latter can be converted into a pseudo-three-dimensional image by adding a slight bias to the vertical sweep according to the magnitude of the time-of-flight signal. BHTV tools typically include means for monitoring the tool orientation within the borehole, such as a fluxgate magnetometer rotating in unison with the transducer. A good technical description of a borehole televiewer suitable for use in a geothermal environment may be found in "Development of a Geothermal Acoustic Borehole Televiewer", by Fred B. Heard and Tom J. Bauman, Sandia Report SAND83-0681, August 1983.
One of the principal and extremely valuable benefits furnished by the BHTV logging tool is the pseudo "visual" image of the borehole wall which it furnishes. Subtleties in the formation, bedding, bedding planes, dip, and so forth, can be observed and studied in a manner unavailable elsewhere. Especially in the oil industry, conventional optical viewing devices do not suffice, in part due to the typically extremely hostile environment, but primarily because the fluid medium in the borehole is normally opaque to optical energy.
As shown in the above-noted publications, borehole televiewers scan radially, typically with a single transducer, essentially looking at a small ring encircling the transducer in the transverse plane thereof. As the borehole televiewer is then moved vertically through the borehole, the path or trail of this ring, as it moves along the borehole wall, in turn describes the wall. This description is then accumulated to generate the displays discussed above.
As will be appreciated, there is with this type of tool, as with most logging tools, the ever-vexing problem of logging speed versus resolution. Logging operations typically need to be accomplished "as quickly as possible", because drilling operations for the borehole which is being logged must be suspended for most logging operations. Such periods of interrupted drilling are very costly and must be kept as brief as possible. This well-recognized problem is addressed, for example, in the above-noted U.S. Pat. No. 4,601,024 (see Col. 10, lines 37-44), which then offers a solution: multiple transducers simultaneously scanning the borehole wall.
Another, and (at first appearances) very much simpler solution is simply to rotae the acoustical transducer faster. The speed of rotation is determined by considering the spot size of the acoustical signal (about 1/3 inch in an 8-10 inch borehole), the pulse rate (e.g., 1500 pulses per second), the rate of rotation (e.g., 3 rps), and the vertical logging rate (e.g., 300 feet per hour), such that substantially 100% of the borehole wall is covered. Three revolutions per second has been a common speed in some tools for a number of years, and more recently tools have been introduced having a speed of six revolutions per second. Due to the small radial dimensions of the borehole and the limits to the resolution attainable by the acoustical transducer itself (the size of the reflected acoustical energy "spot" having finite dimensions), rotational rates for the transducer could be very substantially increased with no loss of resolution from acoustical effects. (There could be a loss of borehole coverage, however, depending upon these parameters and the borehole size.)
That being the case, there remains the question why speeds have not been increased. Data transmission rate limitations could eventually impose limits, but data rates at these speeds are well below saturation. Rather, the major problems have been mechanical. In particular, the viscosity of the fluid, typically motor oil or brake fluid, in which the rotating transducer is bathed becomes a significant limiting factor when rotational speeds are increased.
More particularly, due to the extreme pressures encountered in a borehole, the space surrounding the transducer is filled with liquid rather than air, since air would be too compressible and too attenuating. Due to the extreme operating conditions, this transducer liquid must be rather special. It must provide lubricating properties, should preferably be inexpensive, non-toxic, thermally conductive, electrically nonconductive, non-reactive with borehole televiewer components which it contacts, and it must be tolerant of borehole operating temperatures (high boiling point, e.g., above approximately 300.degree. F.-350.degree. F.) and pressures. It has been found that certain motor oils and brake fluids, as mentioned, meet many of these requirements fairly well.
However, it has also been found, although it is believed that this is not yet recognized in the industry, that the acoustic impedances of such motor oils and brake fluids are very poorly matched to the acoustical window materials used in at least one of the more advanced BHTV tools, and the elastomeric transducer backing material. That is, a portion of the outer body of the tool or sonde housing is typically made of a material substantially transparent to the acoustical energy (pulses) being bounced off the borehole wall. In the ideal situation, the acoustical impedance of the window material substantially matches that of the fluid (drilling mud) in the borehole, and the acoustical impedance of the acoustical fluid surrounding the transducer within the tool matches that of the acoustical window material. Although such an acoustical fluid has been identified and used for several years by the assignee of the present invention, it is believed that other practitioners in this art have not successfully utilized such a fluid.
The acoustical fluid so identified and used is a commercially available synthetic heat transfer fluid, a modified terphenyl, which is available from the Monsanto Company under the registered trademark "Therminol 66".RTM.. It has an acoustic impedance .rho.C=1531 kilorayls. The material used for the window by the assignee of the present invention, also believed to be otherwise unknown to practitioners in this art, has been polymethylpentene, which has an acoustic .rho.C=1820 kilorayls. This is a good match to that of typical borehole fluids (1700-2300 kilorayls).
Acoustically and otherwise, Monsanto's Therminal 66 has worked well in this capacity as the acoustic fluid for coupling the acoustic pulses from the enclosed transducer to the borehole fluid, through the window material in the tool housing. Therminal 66.RTM. is also advantageously non-conductive, lubricating to the moving parts in the tool, and has pressure-dependent flashpoints and boiling points which are beyond the range of normal tool operation. It has a density of 1 g/cc and a sound velocity of 1531 m/sec. Unfortunately, however, it also has a relatively high viscosity at low operating temperatures (approximately 500 cSt at 4.degree. C.). This problem is alleviated somewhat when the oil warms after the televiewer is lowered into the borehole.
However, testing of the tool at the well site before logging is often performed at surface temperatures below the operating temperature range of the mechanical section. The oil must then sometimes be preheated to lower the kinematic viscosity. Otherwise, damage to parts such as the precision speed reducer or other drive components may result from such cold start tests. Faster rotational speeds then become impractical, limited by the motor power consumption and the motor's maximum available torque. Torque load, power consumption, and resultant motor heat generation go up exponentially with increases in viscosity and rotational speed, possibly requiring a larger drive motor and gear drive assembly. Since space and power in logging tools are almost always at a premium, and certainly so with borehole televiewers, it can be appreciated why rotational speeds have remained so low.
A need therefore remains for such a borehole televiewer acoustical fluid which retains desirable acoustical and physical properties of fluids such as Therminol 66.RTM., but which has a materially reduced kinematic viscosity over the full normal temperature range of tool operation.